Tungsten halogen lamps possessing the aforementioned press seal end are known in the art. Examples include:
______________________________________ 3,548,245 Biscoff 3,588,315 Levand, Jr. et al 3,668,456 Anderson 3,753,026 Goorissen 4,110,657 Sobieski 4,254,300 Thompson-Russell RE 31,519.sup. Sobieski ______________________________________
As indicated in the aformentioned patents, it is known in tungsten halogen lamps to utilized a thin molybdenum foil (or ribbon) of substantially rectangular configuration within the press seal end of the lamp's envelope. Typically, this thin foil member is connected (e.g., welded) to a pair of electrically conducting wires to thus form one of the lead-in wire assemblies for the lamp's filament. Understandably, should a singular filament be utilized, two such assemblies are employed. One of these conducting wires (that being connected to or forming part of the filament component) is typically of tungsten material and extends interiorly of the high silica glass envelope of the lamp. The remaining, second conducting wire may be of molybdenum or similar conducting material and extends exteriorly of the envelope.
Understandably, formation of an effective seal about the molybdenum foil and corresponding end portions of the respective conducting wires is extremely important. Should cracks or similar imperfections occur in the area of the foil members, the seal can be broken, resulting in ruination of the lamp product. Crack formation (or at least initial formation followed by subsequent expansion during lamp operation) typically occurs during or shortly after the pressing operation to form the sealed end. During this pressing operation, the glass material is heated to extremely high temperatures (e.g., using oxygen burners) in the vicinity of the eventual seal. Stainless steel pinch (or press) jaws engage the outer surfaces of this heated glass and compress said glass about the already formed conducting wire-molybdenum foil assembly. Typically, gaseous nitrogen is used at this phase of the operation to "cool" the conducting wire-molybdenum foiled assembly. Understandably, such an operation is deemed extremely harsh and can result in crack formation or the like as a result of the substantial difference in coefficients of thermal expansion of the materials utilized. For example, glass material having a silicon oxide content of at least ninety-five percent by weight as used in some lamps of this variety possesses a coefficient of thermal expansion of approximately 10.times.10.sup.-7 per deg. Celsius as compared to the coefficient of thermal expansion of tungsten and molybdenum (45.times.10.sup.-7 and 54.times.10.sup.-7 per deg. Celsius, respectively).
Crack formation is also readily possible during the subsequent phase of lamp formation wherein liquid nitrogen is introduced into the remaining, open end of the glass tubing which eventually forms the lamp envelope. This flushing operation occurs during the exhaust phase of lamp formation and exposes the heated press sealed end of the tubing to such relatively cold material. It is estimated that during the aforementioned press sealing operation, seal temperatures in excess of 1000.degree. Celsius are attained.
It is thus believed that an electric lamp including a press seal end formed in such a manner so as to substantially prevent crack formation therein would constitute a significant advancement in the art.